Types of Evaluation of Crude Drugs

Evaluation of a drug helps identify a drug and determine its purity. The need for evaluation is due to the biochemical variations in the drug, its effect on treatment and storage, and the presence of adulterants and substitutes.

Methods that are currently employed in evaluating drugs are physical, chemical, biological, organoleptic, and microscopic.

Physical Evaluation of Crude Drugs

The physical evaluation of the crude drugs is done with the help of physical characteristics such as

Solubility
Optical rotation
Refractive index
Specific gravity
Melting point
Viscosity
Fiber elasticity
Water content

Chemical Evaluation of Crude Drugs

The chemical evaluation of crude drugs consists of qualitative and quantitative methods. There also are instrumental analysis and chemical assays.

Qualitative Chemical tests

Isolation, purification, and identification of various phytoconstituents form the qualitative evaluation of crude drugs. Alkaloids, tannins, glycosides, etc are identified by these steps. The identification test will be different for each type of constituent.

Specific tests

Van Urk test for ergot alkaloids
Vitali-morin test for tropane alkaloids
Murexide test for purine alkaloids
Iodine for starch

Detection of adulteration

Halphen’s test detects cottonseed oil
Fiehes test for honey
Colophony in resins, waxes, and balsams is identified with copper acetate
Baudouin’s test detects sesame oil in olive oil
Acetic and nitric acids are used for detecting Gurjun balsam in copaiba

Quantitative Evaluation

Quantitative tests include tests for,

Acid value for resins and balsams
Saponification value of balsam
Ester value of volatile oils and balsam
Acetyl value of volatile oils

Chemical Assays

Chemical assays help determine the total content of the active constituents. It also helps determine the presence or absence of an inferior drug so that necessary steps may be taken for a complete substitution of the constituent. A few examples of chemical assays are given below.

Total alkaloid content in belladonna
Total alkaloid and nonphenolic alkaloid in ipecacuanha
Alkaloid strychnine in nux vomica
Resin in jalap
Vitamins in cod liver oil

Instrumental Analysis

Here, the chemical groups are analyzed using chromatographic methods and spectroscopic methods. Paper chromatography, TLC, gas chromatography, HPTLC, HP Liquid chromatography, etc are the common chromatographic methods used.

Different methods of spectroscopy include visible spectroscopy, UV spectroscopy, infrared spectroscopy, mass spectroscopy, nuclear magnetic spectroscopy, etc.

Biological Evaluation of Crude Drugs

The biological evaluation of crude drugs is done to determine their toxicity, potency, and pharmacological activity. This is a far better method of evaluation than chemical and physical evaluation. Biological evaluation helps conclude its biological activity more precisely.

Bioassay or biological evaluation should be simple but must have access to a large number of tests and screening. Bioassay can be tissue or organ method, symptomatic or toxic. While isolated tissue or organs are used for the first type of evaluation, different types of animals are used for symptomatic and toxic evaluation.

In biological evaluation, the required amount of drug is determined for the definite effect in the chosen animals, isolated tissue, or organs under standard conditions. Certain methods use reference standards to reduce possible errors.

Toxicity studies on animals help decide the effective dose and lethal dose of the drugs. Mice, guinea pigs, young domestic chickens, pigeons, cats, worms, rabbits, etc are commonly used animals for the biological evaluation of crude drugs. Animals are chosen according to the drug and its target.

Glycosides from Digitalis are tried in the pigeon bloodstream to observe any lethal effects.
Rabbits are used for checking drugs that affect the eyes.
The effects of Ergot are tried in rabbits in their cock combs, intestine, or uterus.
Guinea pigs are mainly used to study the oxytocic effects of vasopressin.
Oxytocic drugs are injected into young chickens to study their effects on blood pressure.
Mydriartic effects of drugs are studied in cat’s eyes.
Cardiac and gastrointestinal effects of drugs are conducted in dogs.
Anthelmintic drugs are tried in worms.

Microbiological assays determine the effects of drugs on microbes, helping in the identification of antimicrobial drugs. Agar well diffusion, disc diffusion, turbidimetric, etc are the microbiological assay methods. Bacterial yeast molds are used to study vitamins.

Organoleptic Evaluation of Crude Drugs

The evaluation of crude drugs based on the sensory organs is called organoleptic evaluation. The drugs are assessed based on their color, size, shape, odor, texture, taste, etc. It is the simplest form of drug evaluation and the manual process as well.

Such analyses are useful for distinguishing drugs having similar appearances. In those cases, the taste, odor, or texture are assessed to separate one from the other.

Talka gum is brown or colorless while Acacia gum is white or yellow.
Mangosteen fruit has a darker rind with radiate stigmas in a wedge shape.
Bloodroot has a dark reddish-brown color while Hydrastis is yellow.
Ginger and capsicum have their distinct odor.
Chirata and gentian have a bitter taste.
Indian and Alexandrian senna are morphologically different.
Cinnamon has a dark brown color
Volatile oil drugs have a distinctive aroma
Capsicum has a pungent taste while licorice is sweet
Shape as a distinguishing feature is seen in the wavy Rauwolfia, the Ovoid tears in Acacia, and the disc shape of Nuxvomica
The small size of umbelliferous fruits
The silky texture of Nux vomica
Cinchona and Quassia have fractured surface
The general appearance is considered to find the percentage of seeds in colocynth and stalks in clove

Microscopic Evaluation of Crude Drugs

Microscopic evaluation is conducted on small fragments, powdered drugs, and other minute parts of plants that cannot be subject to physical evaluation methods. This is part of the initial identification methods.

This is possible due to the unique anatomical characteristics of plant parts. The cell wall, presence or absence of stomata, cell contents, starch grains, calcium oxalate crystals, lignified or scelrenchymatous cells, etc are all part of this process.

This initial examination helps identify the parts and any possible contamination. For eg., clove stalks will have calcium oxalate crystals and sclereids in them but cloves do not have these two. Another example is the presence of sclerenchyma in Rauwolfia serpentine which will be absent in other species which are adulterants.

Similarly, other drugs and their adulterants are identified with the help of microscopic examination, based on the presence or absence of such features.

References

https://www.jiwaji.edu/pdf/ecourse/pharmaceutical/Evaluation%20of%20crude%20drugs.pdf
Shah, Biren N, Avinash Seth. (2010). Textbook of Pharmacognosy and Phytochemistry. Elsevier.
https://www.pharmacy180.com/article/biological-evaluation-74/
Pharmacognosy & Phytochemistry – I